Okay, I’m not exactly going to write about how to communicate with aliens, but rather, I want to discuss some estimates of how many potentially habitable planets might exist, and likewise, the number of alien civilizations with which we (humans) could potentially communicate. As for Twitter, you’ll see where I’m going with that shortly.
Let’s discuss the number of potentially habitable planets first. First, we have to recognize that the cosmic time scale is very large, and that in the lifespan of the galaxy so far many potentially habitable planets may have already popped into and out of existence. The ones that we want to include are only the ones that currently exist. To estimate how many exist at any given time, we’ll use this formula:
number of habitable planets at a given time = (rate of formation of planets)(average habitable lifetime of a planet)(% of planets that are habitable)
Okay, seems simple enough. All we have to do now is figure out what numbers the three parameters on the right should be. Let’s look at the rate of formation of planets first. Since astronomers haven’t been able to directly observe many exoplanets, there isn’t much data to provide an estimate. However, we can rewrite this parameter as:
rate of formation of planets=(rate of formation of stars)(number of planets per star)
The rate of formation of stars can be estimated as follows:
rate of formation of stars=(number of stars in galaxy)/(lifetime of the galaxy so far)
This of course assumes that the rate of formation of stars has been constant over time since the birth of the galaxy, which is not strictly true, but the difference shouldn’t throw off our estimate too much – at least, not compared to the margin of error on the other parameters we’re about to estimate (eg, the number of stars in the galaxy). Anyway, ballpark estimates for the number of stars in the galaxy and the lifetime of the galaxy both exist, so we can stop there.
As for the number of planets per star – this is more problematic. As mentioned earlier, exoplanets are difficult to observe, and not much empirical data exists on them yet. Certainly not enough to accurately estimate the number of planets per star. However, we do have one sample point – our solar system – from which we can extrapolate. Extrapolation is, of course, a risky enterprise, but in this case there isn’t much alternative. Our solar system has 8 planets (sorry Pluto), but unfortunately, not all solar systems have only one star, so even if we extrapolate that all solar systems with one star average 8 planets, we can’t say the same for solar systems with two or more stars. So we’re going to have to split our estimate for the number of habitable planets into two separate estimates: One for solar systems with one star, and one for binary systems (systems with more than two stars are rare enough that we’ll neglect them).
Our equation for the number of habitable planets at a given time is now becomes slightly more messy, as we include the fraction of non-binary systems and the fraction of binary systems. It’s estimated that 2/3 of systems are single star like ours, and about 1/3 are binary systems. We still don’t have any estimate of the number of planets per binary system or the fraction of binary planets which are habitable, but it is estimated that 50-60% of binary systems do allow for planets to have stable orbits in a habitable zone, so let us assume (based on our own solar system) that every binary system that allows them, has them. This allows us to conservatively replace these two parameters with the fraction ½.
So far we’ve only clarified the first of the three para meters that we’d originally set out to calculate. Fortunately, the second parameter, average habitable lifetime of a planet, won’t be too difficult. Since there’s no data other than our own planet, again we’ll use the Earth and extrapolate. The Earth is about 4.5 billion years old (sorry creationists, using 6000 messes up my equations), life is thought to have started about 0.5 billion years in, and the Earth is expected to remain habitable (not necessarily for humans) for another 2.3 billion years. Thus the habitable lifetime of the Earth is 6.3 billion years – not too bad, considering the universe is only an estimated 13.7 billion years old, and our galaxy is only about 200 million years younger.
The third parameter – the fraction of planets that are habitable – is also relatively straightforward. Most of the solar systems in the galaxy are in a non-habitable zone (too much radiation from neighboring stars) – our own system remains in a habitable zone by staying between the spiral arms of the Milky Way as it revolves. It is estimated that only 1/10th of the solar systems are isolated enough to harbor life. For non-binary systems like our own, we’ll assume that the average number of planets is 8 and the fraction of habitable planets is 1/8 – multiplying them together gives us 1, which makes sense since as far as we know the Earth is the only habitable planet in our solar system. There is speculation about some of Jupiter’s moons, but if we want to include moons we have to go back and open a whole new can of worms. So forget moons for the moment. For binary systems, we already assumed that the number of planets per binary system times the fraction of habitable planets per binary system was ½, so we’ll stick with that.
Before we go any further, I want to address some potential criticisms over taking the fraction of habitable planets in single star systems as 1/8. Obviously this estimate has quite a bit of uncertainty, due to the fact that our solar system is the only data point in our sample. However, some might go even further and say that it is impossible to extrapolate from our own existence that any other planets with life exist, since if we did not exist then we would not be able to ponder such questions in the first place. However, I do not identify with such anthropocentric arguments – historically, anthropocentric arguments have been overturned, and I have no inclination to assume that we occupy any privileged or unique position in the universe simply by mere virtue of our existence.
And now, the moment of truth – our estimate. Let’s be conservative about the number of stars in the galaxy, and say that there are 100 billion. In that case, our estimate for the number of habitable planets currently in existence is (100 billion/13.5 billion)(6.3 billion)(1/10)[(2/3)(8)(1/8)+(1/3)(1/2)]=3.9 billion worlds that could harbor life.
Does that sound too high? If there are so many worlds that could harbor life, why haven’t the aliens made contact? Well, first of all, one should note that a world that could potentially harbor life may not necessarily do so. Second, even if it does harbors life, that life may not be anything other than rudimentary forms of life such as bacteria. Third, even if complex life exists, it may not necessarily be intelligent. Fourth, even if intelligent life exists, it may not be technologically advanced enough to communicate. Or, equally if not more likely is the possibility that the intelligent life may be so technologically advanced that we lack the technology to receive its messages. For instance, it is very likely that a civilization moderately more advanced than ours would send interstellar messages using neutrinos rather than electromagnetic radiation, as neutrinos travel only slightly slower than light but have the advantage of being electrically neutral, which means that neutrino signals would not weaken from interference nearly as much over interstellar distances.
Let’s try to quantify these numbers just a bit, and estimate the number of extraterrestrial civilizations that we could potentially communicate with. First, there doesn’t exist any data on what percentage of worlds that could potentially harbor life actually do so, whether that life eventually becomes complex life, whether that complex life eventually becomes intelligent life, and so on. All we have to go on is what happened here on Earth, and we know that all of these things happened on Earth. So, despite the obvious uncertainty, I will extrapolate that any world that can potentially harbor life does, that rudimentary life eventually becomes complex life, that complex life eventually becomes intelligent, and that intelligent life will always leak electromagnetic radiation into space for some period of time. All of these fractions are therefore set to 1.
However, the human race lacks the technology to communicate with most of the galaxy – our strongest radio signals spread out in space and become too weak to pick up more than a hundred or so light years away. I won’t assume that an alien species will make more of an effort to pick up our signals than we do to pick up theirs – in fact it is rather arrogant to think that an alien civilization would undertake the expense of building arrays of kilometers-wide dishes on the off-chance that they would be able to watch some of our TV shows.
But even supposing that we effectively broadcast our messages up to 500 light years away – the galaxy is 100,000 light years across, which means that the fraction of it that we can communicate with is at most roughly (500^2)/(50,000^2)=0.0001, or 0.01% of the galaxy. And even supposing that there were an alien civilization within our small broadcasting vicinity that wasn’t leaking radiation but was receiving our communications and responding – they would have to be within 35 light-years, or 0.5% of our broadcasting area, for us to have received a response by now. In fact, the percentage should be even smaller – for the galaxy I made the simplifying assumption that the galaxy is basically flat, but for the purposes of our tiny broadcasting range it makes more sense to think of it as a sphere, which means that the alien civilization would have to be within 35^3/500^3=.0003, or 0.03% of the volume of our broadcasting sphere for us to have received their responses. For this reason it is far more likely that we would receive a transmission from an alien civilization sent to anyone capable of listening rather than a directed response.
Here on Earth, no systematic attempt at purposely directing more powerful transmissions to alien civilizations has been made (for reasons which I will soon elaborate), so I will assume that alien civilizations are not likely to expend much effort on such tasks either – that if we were going to receive any signals from alien civilizations they would most likely be unintentionally leaked broadcasts, like the vast majority of ours. Which in turn means that if we were going to pick up any such broadcasts, we almost certainly would have already.
Let’s examine the search for extraterrestrial life from the transmitter’s perspective. If we humans ever decided to construct a transmitter to contact extraterrestrials that could transmit messages that we ourselves could receive, the cost of such an enterprise would be – for lack of a better word – astronomical. At least for trying to send an actual message. Rather of sending actual messages, it would be more economical to construct a Benford beacon – that is, a transmitter that pings (ie, “tweets”) various star systems with short but steady bursts of electromagnetic radiation that are obviously not produced by any natural sources, rather than trying to send actual information, as the latter is much more expensive. Then, in the event that an alien civilization actually picks up on it and focuses in, we could also send an actual message along with it on a sideband at a lower power. Extraterrestrials focused on minimizing costs by using a Benford beacon would be transmitting at frequencies higher than what SETI (Search for Extraterrestrial Intelligence) is currently examining.
It should be noted that even with a Benford beacon, it would cost about $200,000 per light year of coherent pinging – or $20 billion dollars to send a single ping to a star across the galaxy. To send an actual message across the galaxy would cost $7 trillion. Regardless of the conversion to alien currency, somebody would be paying a lot of long-distance fees, and there certainly wouldn’t seem to be much return on an investment sent halfway across the galaxy – for closer distances, perhaps. In any case, a realistic transmitter certainly would not transmit the types of messages that SETI hopes to receive – nor would SETI find the type of message that we ourselves would transmit – which is a rather damning indictment of the SETI program.
The only other parameter that we must estimate is the length of time for which another civilization could communicate with us. We have only been able to broadcast messages to space for about 70 years – less than the blink of an eye from the cosmic perspective. In fact, even if you were watching from the cosmic perspective without blinking, you would still miss it.
One might argue that the human race could potentially be transmitting strong signals into space for centuries to come, but if you look at the trend of recent technologies, we are discovering how to send messages more directly and efficiently, which means that the Earth is moving more and more toward radio silence. Furthermore, our future efforts toward interstellar could very well rely solely on neutrinos or other exotic particles better suited for interstellar communication, rather than electromagnetic radiation. Plus there’s always the possibility that we’ll nuke ourselves to extinction. All of which means that 70 years isn’t an entirely unreasonable estimate of how long an extraterrestrial civilization might be broadcasting the types of messages which we are in a position to receive.
So, keeping the fleeting transience of our existence in mind, along with the depressing limitations of our technology, and applying our calculations from habitable planets to communicating civilizations, we get: (100 billion/13.5 billion)(70)(1/10000)(1/10)[(2/3)(8)(1/8)+(1/3)(1/2)]=0.004 civilizations that we are able to communicate with – which would explain the lack of interstellar conversation. The silver lining is that as the human race advances, people will invent and discover new ways (while perhaps rethinking the old ways) of communicating across the interstellar void, which will increase the possibility of making contact.